1. Field of the Invention
The present invention relates to an antenna and a wristwatch provided with the antenna.
2. Description of Related Art
Currently, a long-wave standard radio wave including the time data or the time code is transmitted in countries (for example, Germany, England, Switzerland, Japan and the like). In Japan, the long-wave standard radio waves of 40 kHz and 60 kHz that are subjected to amplitude modulation by the time code in a predetermined format are transmitted from two transmitting stations (in Hukushima prefecture and Saga prefecture). The time code having a frame with a period of 60 seconds is transmitted every time a minute digit of the correct time is updated, that is, every one minute.
Recently, a watch so-called a radio watch which corrects the current time data by receiving the standard radio wave including such time code has been put to practical use. The radio watch receives the standard radio wave through an antenna which is stored in the radio watch every predetermined time, amplifying and modulating it to decode the time code, and corrects the current time of the radio watch.
As the receiving antenna stored in the radio watch, a bar antenna is generally used. An earlier developed antenna comprises a bar-shaped core which is formed with a magnetic body such as ferrite, amorphous or the like, and a coil which is formed by winding a lead wire such as copper wire or the like around the core.
When the antenna is placed in a magnetic field by the standard radio wave (hereinafter, referred to as a “signal magnetic field”), the magnetic field acts on the antenna as follows. The standard radio wave is an alternating radio wave, so that the segments of the magnetic field is an alternating magnetic field in which the strength or the direction periodically changes.
When the core is placed to make an axis line thereof be parallel to the direction of the magnetic field in the signal magnetic field, a magnetic flux (hereinafter, referred to as a “signal magnetic flux”) by the signal magnetic field is concentrated into the core having a high permeability compared to the surrounding space.
When alternating-current power is applied to the coil of the antenna, a magnetic flux which corresponds to the time change of the alternating current flowing in the coil (that is, direction and strength change) is generated.
Accordingly, when the antenna is placed in the signal magnetic field, the signal magnetic flux is concentrated in the core to pass the coil, and an induced electromotive force V is generated in the coil to generate a magnetic flux (hereinafter, referred to as a “generated magnetic flux”) that opposes the change of the signal magnetic flux in the coil according to Lenz's law. The signal magnetic field is the alternating magnetic field, so that the strength or the direction of the signal magnetic field periodically changes. Accordingly, the induced electromotive force becomes alternating-current power, and the generated magnetic flux becomes an alternating magnetic field which periodically changes the strength or the direction corresponding to the time change of the signal magnetic flux.
The induced electromotive force V generated in the coil is detected by a receiving circuit connected to the coil. The receiving circuit comprises a tuning capacitor Cress and a loss resistance Ra for tuning to the frequency of the standard radio wave desire to receive (40 kHz or 60 kHz).
In the earlier developed antenna (bar antenna) having such structure, the receiver sensitivity of the standard radio wave depends upon the strength of the magnetic field in the coil (that is, magnetic flux density). Therefore, there has been known the antenna in which the sectional area of both end portions of the core (magnetic body) is increased to trap more magnetic flux, thereby improving the receiver sensitivity by making more signal magnetic flux pass through the coil.
However, in the above described earlier developed antenna, it is not avoided to cause loss by the signal magnetic flux.    (1) When a part of the signal magnetic flux passes (crosses) the both end portions of the coil, loss by the signal magnetic flux may be caused.    (2) When a part of the signal magnetic flux passes through the outside of the coil, loss by the signal magnetic flux may be caused or the receiving efficiency may decrease.    (3) When there is a metal near the antenna, in a space including a portion of the metal, loss is caused because a part of the generated magnetic flux passes the metal. That is, when a part of the generated magnetic flux passes the metal, eddy current flows in the metal, so that eddy current loss may be generated. It is considered that the coil and the metal are magnetically coupled with a predetermined coupling coefficient k, and a part of the generated power in the coil (induced electromotive force V) is consumed in the metal, so that the receiver sensitivity of the antenna is reduced.